A digital-to-analog converter (DAC) is an electronic device that is utilized to covert digital data into analog form. As such, digital-to-analog converters are commonly used in a wide variety of environments in order to generate continuous, physical signals, such as signals relating to voltage, current or charge, using digital information.
Digital-to-analog converters typically perform signal conversion by representing an input digital signal into a sequence of nearly constant, analog signals of a fixed duration that is defined by the sample rate of a clock signal. The aforementioned sequence, or step function, of analog signals is then processed using analog filtering techniques to yield a smoothly varying signal.
Presently, there are many different types of DACs available in the marketplace. DACs are commonly categorized, or defined, by several characteristic performance metrics including, inter alia, dynamic range (i.e. signal fidelity or quality), bandwidth (i.e. the range of measurable frequencies), and power consumption.
As can be appreciated, the dynamic range associated with a DAC is influenced by a number of contributing factors including, but not limited to, the number of signal bits as well as the overall fidelity of signals relative to noise, distortion and interference. The dynamic range (DR), or dynamic performance, of a DAC is often quantified in terms of its effective number of bits (ENOB), which represents the number of bits for each measure that are, on average, not noise.
The bandwidth associated with a DAC is also influenced by a number of contributing factors including, among other things, the sample rate of the input signal that the device can handle. In particular, the bandwidth associated with a DAC is generally limited to one-half the input sample rate in view of the Nyquist sampling theorem, which states at least two samples per cycle are required to ensure proper signal reconstruction (i.e. the sampling rate must be at least two times the bandwidth of the signal).
It has been found that conventional DACs suffer from a notable drawback. Specifically, when DACs are utilized in connection with signals of relatively broad bandwidth (i.e. signals of faster speeds), a correspondingly high sample rate is required in view of the Nyquist sampling theorem. For example, a 10 GHz instantaneous bandwidth output signal would require an input sample rate of at least 20 Gsamples/sec.
Accordingly, when required to handle signals of a higher frequency, or broader bandwidth, the corresponding increase in the sampling rate causes a digital-to-analog converter to operate in a less accurate manner due to natural imperfections in the physical device. The inaccuracies associated with the operation of conventional DACs when handling faster, high frequency signals often yield a dynamic range that is unacceptably low and, as a result, problematic.
FIG. 1 is a graph depicting the effective dynamic range of a selection of conventional DACs in view of intended signal bandwidth. It is to be understood that the selection of conventional DACs shown in FIG. 1, although not entirely inclusive, nonetheless provides a reflective landscape of DACs presently available in the marketplace.
As can be readily seen in FIG. 1, most conventional DACs exhibit a higher dynamic range in connection with lower bandwidth signals. However, when conventional DACs are designed to handle higher bandwidth signals, the performance characteristics associated with the DACs tend to drop, as evidenced by the corresponding decrease in dynamic range.
In view thereof, various techniques have been applied in the art to enable digital-to-analog converters to produce high bandwidth output signals at an acceptable dynamic range level.
One technique utilized to enable DACs to produce high bandwidth output signals at acceptable dynamic range levels is to simply manufacture a higher quality DAC through the use of, among other things, improved fabrication processes, higher performing materials, and more complex circuitry that includes embedded error correction. However, it has been found that the above-identified techniques substantially increase the overall manufacturing costs associated with the DAC, which is highly undesirable.
Another technique utilized to enable DACs to produce high bandwidth output signals at acceptable dynamic range levels is to combine together multiple DACs that share a common, lower sample rate. For instance, the aforementioned technique is often implemented through the use of (i) multiple, lower rate DACs with time staggered samples, (ii) multiple DACs, with each DAC designated to cover a particular bandwidth of operation, or (iii) multiple DACs followed by upconversion to different bandwidths. However, it has been found that the above-identified techniques tend to experience significant limitations relating to output bandwidth, dynamic range, size requirements, and/or power consumption (e.g. due to errors inherent in designing and manufacturing parallel circuits, such as output signal mismatch effects). Moreover, these configurations, which aggregate multiple, lower sample rate DACs, incur a penalty of errors due to an imperfect alignment, or interleaving, of the output signals from the various DACs (e.g. with respect to time, bandwidth, amplitude, phase, etc.). In particular, it has been found that signal alignment errors are most prevalent when a large number of DACs are aggregated.